1. Field of the Invention
The present invention relates to an ethylene/polycyclic monomer random copolymer, a process for the preparation of this random copolymer, and an optical article formed by molding this random copolymer.
This random copolymer has an excellent transparency, is optically homegeneous, has a small birefringence, and a well-balanced heat resistance, chemical resistance, dimensional stability, rigidity and other mechanical properties. This copolymer is valuable for use in the manufacture of an optical disk, an optical fiber, an optical filter, a plastic lens and the like.
2. Description of the Related Art
Recently, transparent resins have been used as various optical materials, and the fields of application of transparent resins as information-recording materials in particular have been broadened since the mass production of these transparent resins became possible.
Currently, a polycarbonate resin and a polymethyl methacrylate resin are mainly used as the material of an optical disk substrate. As the polycarbonate resin has a high glass transition temperature (Tg), and thus the polycarbonate resin has a high heat resistance, good transparency, dimensional stability, and mechanical properties. Nevertheless, since the hygroscopicity of the polycarbonate resin is relatively high and the birefringence is large, the optical anisotropy of the polycarbonate resin is large. Furthermore, the polycarbonate resin is weak with regard to an alkali, and the polycarbonate resin tends to undergo hydrolysis. On the other hand, the polymethyl methacrylate resin has excellent optical properties, for example, the polymethyl methacrylate resin has a high transparency and a small birefringence. Moreover, the polymethyl methacrylate has good mechanical properties. However, since the hygroscopicity of the polymethyl methacrylate resin is high, a substrate formed of this resin is easily corroded, and warping often occurs in the disk surface owing to a change of the dimension or shape. Furthermore, since the glass transition temperature (Tg) of the polymethyl methacrylate is about 100.degree. C., distortion at a high temperature cannot be neglected. Still further, the polymethyl methacrylate has a poor solvent resistance.
A resin material having well-balanced transparency, optical properties, heat resistance, chemical resistance, dimensional stability and mechanical properties has not heretofore been known.